Pyrolytic carbon, also called pyrocarbon, is a strong, heat-resistant form of carbon obtained as a product of hydrocarbon pyrolysis. Heart valve prostheses, blood pumps, and other biomedical systems commonly include components consisting of or coated with pyrolytic carbon because it is thromboresistant, as well as strong. Pyrolytic carbon is usually formed by deposition onto an object or substrate by thermally decomposing gaseous hydrocarbons or other carbonaceous substances in vaporous form in the presence of the object.
An apparatus for coating objects with pyrolytic carbon is described in U.S. Pat. No. 4,546,012, the disclosure of which is incorporated herein by reference. Such apparatus generally includes a tubular chamber having a conically shaped lower end through which an upwardly flowing gas stream comprising a mixture of inert gas and gaseous hydrocarbons is admitted through an inlet port at the apex of the cone. Submillimeter particles partially fill a lower portion of the chamber and form a fluidized bed as the upwardly flowing gas stream causes the particles to travel upwardly in the central region of the chamber and then downwardly along the outer perimeter of the chamber. Objects to be coated are immersed in the bed of particles and are levitated along with them while being exposed to the gas stream.
The tubular chamber is heated to about 1300.degree.-2000.degree. C. so that the gaseous hydrocarbons decompose as they permeate through the fluidized particle bed and deposit carbon on the objects to be coated, as well as on the particles. The upper end of the chamber opposite the conical lower end includes an outlet port through which the inert gas and decomposed gaseous hydrocarbons are withdrawn.
Components coated with pyrolytic carbon commonly require machining in order to conform such components to prescribed dimensions. For example, prosthetic heart valves of the type described in U.S. Pat. Nos. 4,254,508, 4,276,658, and 4,328,592 generally include valve bodies, sometimes referred to as orifice rings, which are typically formed of materials, such as pyrocarbon or pyrocarbon-coated graphite. The valve bodies are typically designed to be deformed sufficiently to provide clearance to insert occluders or leaflets which will open and close in response to hydrodynamic pressure and allow blood to flow in only one direction through the valve. After insertion of the leaflets, the valve bodies are allowed to return to an unstressed, annular configuration.
However, it is desirable to provide structure for increasing the stiffness of the orifice ring after the leaflets are installed which permits thinner valve bodies to be used. Such structure is commonly provided by a metal stiffener ring generally held within a shallow annular groove formed in the outer circumferential surface of the orifice ring by an interference fit. The combination of the stiffener ring and the orifice ring provides an assembly having much greater stiffness and resistance to deformation than the orifice ring alone.
The annular groove is formed initially in the substrate but the surface of the pyrolytic carbon coating of the orifice ring is thereafter ground to precise dimension. There is also finish grinding required generally throughout the pivot regions and at the sealing surfaces. Moreover, in making an all-pyrocarbon valve component, there is initial machining after formation, followed by finish grinding after the substrate has been abraded and/or ground away, as generally described in European Patent 0 055 406B. Many valve bodies undergo controlled warpage during coating, and they may require additional grinding to meet tolerances. Following such grinding operations, failures often occur when installing leaflets because the pyrolytic carbon structures are destroyed or rendered unusable by cracking. Moreover, they are susceptible to the formation of microcracks which do not result in immediate destruction but which may worsen and possibly shorten valve lifetime. These components are expensive to manufacture, and failures and reject rates of even a few percent is troublesome and uneconomical. Because of the many critical life-sustaining applications where pyrolytic carbon is used, a decrease in the failure or rejection rate would not only reduce manufacturing costs, but would increase confidence in the integrity of such components.
A present technique for overcoming warpage of pyrolytic carbon is to design the substrates to be coated with built-in distortion which ultimately results in a pyrolytic carbon-coated component of the desired dimensional configuration following coating. However, intentionally designing distortion into a component may further amplify the problem of cracking.
It is now felt that breakage of a pyrolytic carbon structure upon deformation to install leaflets after being machined results from the redistribution and concentration of stresses in the material. When material is removed from the pyrocarbon structure, localized stresses may be created within the pyrolytic carbon which thereafter result in cracking when the valve body is significantly deformed to install the leaflets. The limiting factor for brittle materials such as these is the maximum tensile stress, which for pyrocarbon having about 7 weight percent silicon doping is about 40-50 thousand psi.
Clearly there is a need for a method which eliminates or reduces cracking of pyrolytic carbon structures when deformed after their being machined, and research to discover such a method has continued.